Pulled the latest SL 6 fasbugs, which updated (amongst other packages):

qt-4.6.2-28.el6_5.x86_64
qt-devel-4.6.2-28.el6_5.x86_64
qt-sqlite-4.6.2-28.el6_5.x86_64
qt-x11-4.6.2-28.el6_5.x86_64

Then found that when trying to click on the task bar in KDE to change
tabs the tabs (I don't have tab grouping set) sometimes the tabs
change order, sometimes one takes up the whole screen (masking the
taskbar).

Closing application windows failed to bring back the task bar.
CTRL-ALT-DEL brings the logout options - I click logout and am
returned to the login manager.  Logging in again and problem is
reproducable.  Have tried restarting X-server and a full reboot,
problem persists.

Have done an rpm -ivh --force to the older versions in the SL 6.5 install tree:

qt-4.6.2-26.el6_4.x86_64
qt-devel-4.6.2-26.el6_4.x86_64
qt-sqlite-4.6.2-26.el6_4.x86_64
qt-x11-4.6.2-26.el6_4.x86_64

and clicking tabs in the taskbar to change windows now works in a
stable way again.